High-integration MEMS relative gravimeter

Abstract

The application discloses a high-integration MEMS relative gravity instrument, which is integrated with a gravity sensitive unit, a temperature sensing unit, a heating resistor, an out-of-plane tilt sensing unit, an in-plane tilt sensing unit, an air pressure sensing unit and a vibration sensing unit; the temperature sensing unit senses the temperature change of the MEMS relative gravity instrument; the heating resistor heats the gravity sensitive unit to realize temperature control of the gravity sensitive unit; the out-of-plane tilt sensing unit senses the out-of-plane tilt size of the gravity sensitive unit; the in-plane tilt sensing unit senses the in-plane tilt size of the gravity sensitive unit; the air pressure sensing unit senses the air pressure change, and assists in air pressure control or realizes air pressure effect deduction; and the vibration sensing unit senses the environmental vibration, and assists in vibration isolation or realizes vibration signal deduction. The application realizes accurate control of the sensitive direction of the gravity sensitive unit and the tilt sensing unit by using micro-processing precision, which is favorable for reducing additional clamping components, lowering the direction control difficulty and improving the control precision.

Description

Technical Field

[0001] This invention belongs to the field of instrumentation technology, and more specifically, relates to a highly integrated MEMS relative gravimeter. Background Technology

[0002] The Earth's gravitational field is one of the most fundamental physical fields in near-Earth space, comprehensively reflecting the distribution of Earth's matter and its changes over time and space. It is an indispensable source of information in geodesy, solid earth geophysics, and oceanography. Accurate measurement of the Earth's gravitational field is crucial for regional gravity research and gravity mapping, oil and gas and mineral exploration, gravity-assisted navigation, groundwater and environmental research, volcanology, and the study of geological hazards such as urban subsidence areas. A gravimeter is an instrument for precise gravity measurement, typically containing a gravity sensing unit, a temperature control and compensation system, a tilt control and compensation system, and a pressure control system. The need for high sensitivity and the complex system composition result in gravimeters being relatively large and expensive, making them unsuitable for cost-sensitive applications such as deep well gravity measurement, man-portable gravity measurement, or the deployment of high-density gravity measurement points.

[0003] To achieve high-sensitivity and high-stability gravity measurements, a gravimeter includes both a gravity-sensing unit for gravity sensing and a system to suppress environmental disturbances such as temperature, air pressure, and tilt. To reduce cost and size, the University of Glasgow pioneered the use of MEMS technology to fabricate a silicon-based gravimeter chip, enabling the observation of Earth's gravity tides and validating the application potential of MEMS technology in precision gravity measurement (Nature 531, 614–617, 2016). Subsequently, researchers both domestically and internationally conducted related research, including CN107092038B, Microsystems & Nanoengineering, 5(60), 2019, and Microsystems & Nanoengineering, 2019, 5(1), 1-11. Although these research results successfully reduced the size and cost of the gravity-sensing unit, other components such as temperature control and tilt control still utilize traditionally machined commercial parts, and the overall size and cost of the instrument are not significantly improved compared to typical commercial gravimeters. In addition, using separate components also presents challenges such as the difference between the temperature of the thermometer and the temperature of the gravity sensing unit chip, and installation difficulties caused by aligning the inclinometer and the gravity sensing unit. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a highly integrated MEMS relative gravimeter, which aims to solve the technical problems of low integration, low accuracy, large size and high cost of existing MEMS gravimeter environmental disturbance suppression systems.

[0005] This invention provides a highly integrated MEMS relative gravimeter, which integrates a gravity sensing unit, a temperature sensing unit, a heating resistor, an out-of-plane tilt sensing unit, an in-plane tilt sensing unit, a pressure sensing unit, and a vibration sensing unit. The temperature sensing unit senses temperature changes in the MEMS relative gravimeter; the heating resistor heats the gravity sensing unit to control its temperature; the out-of-plane tilt sensing unit senses the magnitude of the out-of-plane tilt of the gravity sensing unit; the in-plane tilt sensing unit senses the magnitude of the in-plane tilt of the gravity sensing unit; the pressure sensing unit senses pressure changes to assist in pressure control or to subtract pressure effects; and the vibration sensing unit senses environmental vibrations and assists in vibration isolation or to subtract vibration signals.

[0006] This invention enables the miniaturization of the entire MEMS gravimeter, thereby improving instrument stability and meeting the needs of cost-sensitive applications such as deep well gravity measurement, individual soldier-carried gravity measurement, etc., where size and weight requirements are high or high density gravity measurement points need to be arranged.

[0007] Furthermore, the gravity-sensitive unit's sensing structure is a spring-oscillator structure or a quasi-zero stiffness structure; the gravity-sensitive unit is fabricated on a silicon substrate using bulk silicon or SOI processes. The thickness of the silicon structure used to fabricate the gravity-sensitive unit is 50 μm to 1000 μm.

[0008] Furthermore, the temperature sensing unit is made of a material whose resistivity changes with temperature. The material used to make the temperature sensing unit can be titanium, gold, or platinum. The temperature sensing unit can be integrated into the surface of the gravity-sensitive unit in the form of a thin film.

[0009] Furthermore, both the out-of-plane tilt sensing unit and the in-plane tilt sensing unit are MEMS accelerometers with their sensing axes in the horizontal plane, and the out-of-plane tilt sensing unit and the in-plane tilt sensing unit are perpendicular to each other.

[0010] Furthermore, the pressure sensing unit is fabricated using a Pirani gauge made by depositing titanium or gold metal thin film wires, and the pressure is monitored by the temperature change caused by the change in convective heat transfer of the energized thin film resistance under different pressures.

[0011] Furthermore, the vibration sensing unit is an acceleration sensor with its sensitive axis aligned with the direction of gravity.

[0012] Compared with the prior art, the present invention has the following advantages based on the above-described technical solutions:

[0013] (1) In addition to the gravity sensing unit being made using MEMS technology, the environmental influence suppression modules such as temperature, air pressure, and tilt are also made using MEMS technology, which reduces the size of the entire gravimeter system and lowers the cost, showing a significant advantage over existing commercial gravimeters in terms of size and cost.

[0014] (2) The integration of gravity, tilt, temperature, tilt sensing units, and temperature control actuators on a single chip offers the following advantages: (a) Utilizing micromachining precision enables accurate control of the sensing directions of the gravity and tilt sensing units, reducing the need for additional clamping components, simplifying direction control, and improving control accuracy. (b) Compared to combining the gravity sensing unit with a separate temperature sensing unit, integrating them onto a single chip reduces the difference between the temperature sensed by the temperature sensing unit and the actual temperature of the gravity sensing unit, improving temperature effect suppression accuracy. (c) Integrating the temperature control actuator and gravity sensing unit onto a single chip increases the temperature control bandwidth, enabling faster response to temperature changes. (d) The integration of the pressure sensing unit facilitates accurate reflection of internal system pressure changes, avoids pressure monitoring discrepancies caused by inherent limitations in the placement of discrete components, and facilitates the elimination of pressure change effects, thereby improving instrument detection accuracy. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a highly integrated MEMS relative gravimeter provided in an embodiment of the present invention.

[0016] Figure 2 This is an embodiment of the high-integration MEMS relative gravimeter fabrication method provided by the present invention. It should be noted that this example is used to illustrate the feasibility of the present invention and is not a fabrication scheme protected by the present invention.

[0017] The meanings of the symbols in the attached diagram are as follows: 1 is a gravity sensing unit, 2 is a temperature sensing unit, 3 is an out-of-plane tilt sensing unit, 4 is an out-of-plane tilt sensing unit, 5 is an in-plane tilt sensing unit, 6 is a pressure sensing unit, 7 is a vibration sensing unit, 8 is silicon dioxide, 9 is silicon, 10 is a metal layer (typically titanium or gold), 11 is a wet-etched spring, and 12 is a deep silicon etched spring. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] This invention provides a single-chip integrated relative gravimeter system; in addition to the gravity sensing unit, the temperature sensing and control unit, the air pressure sensing unit, and the tilt sensing unit are all integrated on the same chip using microfabrication methods, thereby significantly reducing the size of the relative gravimeter system, reducing costs, and obtaining a miniaturized gravimeter.

[0020] Figure 1 The structure of a single-chip integrated relative gravimeter system provided in an embodiment of the present invention is shown. The chip integrates one or more sensors or actuators selected from the following: gravity sensing unit 1, temperature sensing unit 2, heating resistor 3, out-of-plane tilt sensing unit 4, in-plane tilt sensing unit 5, air pressure sensing unit 6, and vibration sensing unit 7. The temperature sensing unit 2 is made of a material whose resistivity changes with temperature, such as titanium or gold, and is integrated as a thin film on the surface of the gravity sensing unit. Furthermore, a thin film heating wire assembly made of the same material is used to achieve on-chip integrated temperature control. The out-of-plane tilt sensing unit 4 and the in-plane tilt sensing unit 5 are both MEMS accelerometers with their sensing axes in the horizontal plane and are perpendicular to each other, and can be spring-type or resonant-type. The air pressure sensing unit 6 is a Pirani gauge made by depositing metal thin film wire assemblies such as titanium or gold, and monitors air pressure by utilizing the temperature change caused by the change in temperature due to the convective heat transfer of the energized thin film resistor under different air pressures. The vibration sensing unit 7 is an accelerometer whose sensing axis is along the direction of gravity. In addition to these components, GPS signal transmitting and receiving units, wireless communication signal transmitting and receiving units, altitude sensors, and instrument height sensors can also be integrated. These MEMS sensing units can be integrated onto one or more chips in different combinations, ultimately realizing a highly integrated MEMS relative gravimeter.

[0021] The gravity sensing unit is fabricated on a silicon substrate using bulk silicon or SOI processes. The thickness of the silicon structure used to fabricate the gravity sensing unit is between 50μm and 1000μm. The sensing structure of the gravity sensing unit is a spring-oscillator structure or a quasi-zero stiffness structure. The gravity sensing unit integrates a temperature sensing unit 2 to detect changes in chip temperature. The temperature sensing unit is made of a material whose resistivity changes with temperature, typically gold or platinum. The temperature data sensed by the temperature sensing unit is used to suppress the temperature effect of the gravity sensing unit. A heating resistor 3 integrated on the gravity sensing unit is used to heat the gravity sensing unit, achieving temperature control. An out-of-plane tilt sensing unit 4 is integrated on the gravity sensing unit to sense the magnitude of the out-of-plane tilt. An in-plane tilt sensing unit 5 is integrated on the gravity sensing unit to sense the magnitude of the in-plane tilt of the gravimeter chip. The output of the tilt sensing unit integrated on the gravity sensing unit is used to guide mechanical leveling or to achieve tilt compensation. The gravity sensing unit integrates a barometric pressure sensing unit 6 to detect changes in barometric pressure, assisting in barometric pressure control or reducing barometric pressure effects. The gravity sensing unit also integrates a vibration sensing unit 7 to detect environmental vibrations and assist in vibration isolation or reduce vibration signals. The chip is connected to the circuit via wire bonding; the circuit can be composed of one or more PCB boards, ASIC chips, or a combination of thick-film circuits.

[0022] To achieve the integration of multiple sensors on a single chip, the following fabrication process is adopted: Figure 2 As shown, firstly, a metal layer is fabricated on a silicon wafer with SiO2 deposited on one side using wet etching or lift-off processes. This metal layer is used to form the displacement detection electrodes of the accelerometer, the thermistor wires of the barometer, the temperature sensing thermistor, temperature control wire assembly, metal leads, bonding pads, signal lead points, etc. Next, the SiO2 is etched using dry etching or wet etching processes to form a patterned insulating layer. Then, the bulk silicon is processed using dry or wet etching processes to form the spring of the accelerometer, the cavity on the back of the barometer, and other structures. Finally, the chip is connected to a cover plate with electrodes using a bonding process.

[0023] It is worth noting that the gravimeter chip can also be integrated using the 3D stacking bonding method, and should be included within the scope of this patent.

[0024] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A highly integrated MEMS relative gravimeter, characterized in that, The MEMS relative gravimeter integrates a gravity sensing unit (1), a temperature sensing unit (2), a heating resistor (3), an out-of-plane tilt sensing unit (4), an in-plane tilt sensing unit (5), a pressure sensing unit (6), and a vibration sensing unit (7). The gravity-sensitive unit (1) has a spring-oscillator structure or a quasi-zero stiffness structure. The temperature sensing unit (2) is used to sense the temperature change of the MEMS relative to the gravimeter, and the temperature sensing unit (2) is integrated in the form of a thin film on the surface of the gravity sensing unit (1). The heating resistor (3) is used to heat the gravity sensing unit (1) to achieve temperature control of the gravity sensing unit (1), and the heating resistor (3) is in the form of a thin film; The out-of-plane tilt sensing unit (4) is used to sense the out-of-plane tilt magnitude of the gravity sensing unit (1); The in-plane tilt sensing unit (5) is used to sense the magnitude of the in-plane tilt of the gravity sensing unit (1); The pressure sensing unit (6) is used to sense pressure changes, assist in pressure control, or achieve pressure effect deduction. The vibration sensing unit (7) is used to sense environmental vibrations and assist in vibration isolation or to achieve vibration signal deduction. The process for integrating multiple sensing units includes: first, fabricating a metal layer on a silicon wafer with SiO2 deposited on one side using wet etching or stripping processes; then, etching the SiO2 using dry etching or wet etching processes to form a patterned insulating layer; next, processing the bulk silicon using dry or wet etching processes; and finally, connecting the chip to a cover plate with electrodes using a bonding process.

2. The MEMS relative gravimeter as described in claim 1, characterized in that, The gravity-sensitive unit (1) is fabricated on a silicon substrate using bulk silicon or SOI processes.

3. The MEMS relative gravimeter as described in claim 2, characterized in that, The thickness of the silicon structure used to fabricate the gravity-sensitive unit (1) is 50 μm to 1000 μm.

4. The MEMS relative gravimeter as described in claim 1, characterized in that, The temperature sensing unit (2) is made of a material whose resistivity changes with temperature.

5. The MEMS relative gravimeter as described in claim 4, characterized in that, The material used to make the temperature sensing unit (2) is titanium, gold or platinum.

6. The MEMS relative gravimeter as described in any one of claims 1 to 5, characterized in that, Both the out-of-plane tilt sensing unit (4) and the in-plane tilt sensing unit (5) are MEMS accelerometers with their sensing axes in the horizontal plane, and the out-of-plane tilt sensing unit (4) and the in-plane tilt sensing unit (5) are perpendicular to each other.

7. The MEMS relative gravimeter as described in any one of claims 1 to 5, characterized in that, The pressure sensing unit (6) is fabricated by depositing titanium or gold metal thin film wires to create a Pirani gauge, and monitors the pressure by utilizing the temperature change caused by the change in convective heat transfer of the energized thin film resistance under different pressures.

8. The MEMS relative gravimeter as described in any one of claims 1 to 5, characterized in that, The vibration sensing unit (7) is an acceleration sensor whose sensitive axis is along the direction of gravity.

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